Insights on the Behavior of Imidazolium Ionic Liquids as Electrolytes in Carbon-Based Supercapacitors: An Applied Electrochemical Approach

Abstract

This study aims to increase the knowledge on the interactions that occur at the electrode/electrolyte interface in carbon-based electric double layer capacitors (EDLC) when solventfree ionic liquids are used as electrolytes. Many previous studies found in the literature are conducted using theoretical approaches and they are unable to model all the variables and the complexity of an actual device with a complex carbon surface and an ionic liquid (IL). Here, the compatibility between imidazolium ionic liquids and different carbon materialsan activated carbon (AC), a mesoporous carbon (MES), multi-walled carbon nanotubes (MWCNT), and reduced graphene oxide (RGO) -is empirically investigated applying synchronous chronopotentiometric tests to various symmetrical EDLCs. The study of the simultaneous evolution of the cell and electrode potentials of the various carbon/ILs cells, monitoring the evolution of specific capacitances and electrical resistances for each independent electrode, allows inferring about the ion-electrode compatibility, the limiting factors for charge accumulation and its impacts on the performance of the global cell. The results indicate that the sp 2 structures of MWCNT and RGO favor interactions with the EMI + cation on the negative electrode.In the positive electrodes, MES and AC favor interactions with the BF4and TFSIanions, respectively, yielding a higher specific capacitance and lower resistance.

Ionic Liquid Electrolytes for Electrochemical Energy Storage Devices

“…Recently, Ortega et al investigated ion–electrode compatibility based on four different carbon-based electrodes, such as AC, mesoporous carbon (MES), multi-walled carbon nanotubes (MWCNTs), and reduced graphene oxide (rGO) [ 131 ]. They insisted that considering only pore and ion size for choosing the carbon electrode is insufficient.…”

“…A eutectic mixture is defined as a mixture of organic salt and other compounds (urea or chlorine) that inhibit the crystallization of one another at certain ratios, resulting in a decrease in melting point [ 132 ]. It was demonstrated that the crystallization process is mainly affected by anions [ 88 , 130 , 131 , 133 , 134 , 135 ]. Simon et al were able to extend the temperature range from −50 °C to 100 °C by mixing [PIP 13 ][FSI] and [Pyr 14 ][FSI] [ 89 ].…”

“… Specific cell capacitance for the EDLCs with different porous carbon electrodes and with ( a ) EMITFSI and ( b ) as electrolytes; ( c ) Ragone plot for all evaluated EDLCs. Reprinted 2020 from [ 131 ] with permission from the American Chemical Society. …”

Electrical Double‐Layer Capacitors Based on a Ternary Ionic Liquid Electrolyte Operating at Low Temperature with Realistic Gravimetric and Volumetric Energy Outputs

“…In the case of ILs, such as [PMPyrr][TFSI] and [EMIm][TFSI], and using a typical microporous carbon applied for EDLCs, anions enter the carbon pores of the positive electrode during the cell charging, and a smaller number of cations is repelled, while a significant number of anions is ejected from the pores of the negative electrode and a smaller number of cations enter its pores [29] . The charging process depends on both the electrolyte and carbon texture, as elucidated by the studies on ion‐electrode compatibility using various carbonaceous materials and imidazolium based ILs [5c] . In the case of a microporous carbon, and using [EMIm][TFSI] as electrolyte, the positive electrode displayed higher capacitance than the negative one ( C + > C − ), whereas the outcome was reversed when employing [EMIm][BF 4 ] ( C + <C − ).…”

Tuning the Nanoporous Structure of Carbons Derived from the Composite of Cross-Linked Polymers for Charge Storage Applications

“… 15 Leaving aside the discussion related with a particular probe (e.g., nitrogen) for estimating surface and pore sizes, one would still argue over the importance of pore and ion size compatibility, which is further evidenced when ionic liquids are used as electrolytes for charge storage in nanoporous carbons. 16 , 17 More importantly, it has been found that the majority of pores of carbon are underused due to their small size compared to the effective size of ions as well as their orientation while entering the pore. 18 …”